Yes. You are right. The native ReaderBooklet supports PDF, Mobi and Azw document types via a plugin mechanism just like Yifanlu's EPUBBook implementation. But as there are already standalone epub readers like CoolReader and Kindlepdfviewer for Kindle why bother re-implementing another one. What we want is just launching an external reader from native home screen and returning to home screen after we close the document. It's much easier to implement a booklet as a launcher than a real book viewer. And it's already done for Kindle Paperwhite. See the project KPVBooklet at https://github.com/chrox/KPVBooklet.

I didn't suggest to reimplement whole viewer as reader plugin, just launcher part (invoking KPV for various files). But it seems, current way is working good enough and, maybe, reader plugin infrastructure doesn't provide any gains for simple launcher, so congratulations!